Organic resin-covered metal sheets obtained by covering metal sheets such as of aluminum with an organic resin have long been known as materials for producing cans. It has also been known to subject the above organic resin-covered metal sheets to the draw working or the draw-ironing working to produce seamless cans for containing beverages, or to subject the metal sheets to the press-forming to produce can lids such as easy-to-open ends. For example, an organic resin-covered metal sheet having, as an organic resin film, a thermoplastic resin film of a polyester resin comprising chiefly an ethylene terephthalate unit, has been widely used as a material for producing seamless cans (patent document 1).
Further, as a metal sheet used for the preparation of the organic resin-covered metal sheet that is used for producing can bodies and can lids, there is usually used a surface-treated metal sheet of which the surfaces are subjected to the surface treatment such as conversion treatment in order to impart corrosion resistance and close adhesion to the organic resin films. As the surface treatment, there can be exemplified a chromic phosphate process. Namely, the organic resin-covered surface-treated metal sheet comprising the surface-treated metal sheet after chromic phosphate process has been widely used for forming the seamless cans and the like owing to its excellent adaptability to producing cans and excellently adhering property (hot water-resistant adhering property) between the organic resin film and the metal base material during the sterilization treatment after the can has been filled with the content and has been sealed. From the standpoint of protecting the environment, however, it has now been increasingly demanded to treat the surfaces without using chromium.
So far, a number of chromium-free surface treatments have been proposed for materials for producing cans. For producing organic resin-covered seamless aluminum cans, for example, there has been proposed a conversion treatment of the organic/inorganic composite type using a zirconium compound, a phosphorous compound and a phenol compound providing excellent adaptability to producing cans and expressing hot water-resistant adhering property (patent document 2). However, the surface treatment proposed above is a surface treatment of the conversion type (reaction type) that requires the washing with water after the treatment. Namely, this surface treatment produces large quantities of waste water and requires cost for treating the waste water yet exerting large load on the environment.
For the materials for producing cans, on the other hand, there has also been proposed a chromium-free surface treatment of the application type (application type treatment) which, unlike the surface treatment of the conversion type, requires no washing with water after the treatment and, therefore, reduces the cost for treating the waste water and, besides, exerts reduced burden on the environment. For instance, there has been proposed a resin-coated aluminum sheet forming an application type underlying coating that contains a zirconium compound and a poly-acrylic acid crosslinked with zirconium (patent document 3).